A long-standing problem in the linear-beam microwave tube art has been the limitations on microwave tube performance and longevity imposed by the inevitable imperfections in manufacturing tolerances and imprecisions in the electron beam optics at least partially resulting therefrom. Over the years many schemes have been implemented to compensate such problems, for example, to correct beam misalignments or improve beam convergence, and manufacturing techniques have improved to provide better tolerances. However, the more exacting requirements of advanced tube designs of recent years have in many cases outstripped the ability of prior expedients to cope with beam optics and tolerances imperfections. At the same time, the demand for improved efficiences has further exacerbated the problem, since for best efficiency, the diameter of the electron beam within the linear beam tube should desireably approach that of the beam tunnel defined within the tube internal structures through which the beam travels and interacts with microwave energy. In practice, however, the beam diameter must be held to a conservative fraction of the tunnel diameter. Otherwise, the inevitable variations in beam or tunnel diameter from one production tube to another would create an unacceptably high risk of failure due to excessive interception of beam electrons by the surrounding interaction structure.
The advent of linear beam tubes operating at millimeter wavelengths, say above 30 GHz, and at high power, has further exposed the inadequacies of the prior art and increased the need to escape beam optics and tolerances problems. Such millimeter wave tubes have become very important for addressing such needs as high resolution radar to detect previously unresolvable targets, but have not fully realized their potential due to the limited power levels available at reasonable tube system overall weights and sizes. For example, the highest power klystron recently available at 35 GHz has been a 1 KW CW tube requiring solenoid focusing, hence a solenoid power supply, and liquid cooling.
However, permanent-magnet-focused tubes have lagged in power output and efficiency due the far greater physical size constraints due to the small millimeter wavelengths involved, and the consequently greatly-increased effect of the inevitable beam optics and tolerance problems. In a tube for millimeter waves, the beam tunnel, for example, can often be under 30 mils in diameter. At such dimensions, it becomes much more difficult to employ efficient beam-to-tunnel diameter ratios, and the effects of any beam scalloping (variations in beam diameter with distances) are much more serious and likely to lead to an unacceptable degree of interception of beam electrons.
Furthermore, in recent years it has become desireable to utilize electron beams with very great current and power densities as a means to obtain high power outputs despite the need to adhere to conservative design and beam-to-tunnel-diameter ratios, and as a means to help overcome certain other limitations as well. Recently tubes have been devised with beam current densities in excess of 1000 amps per square cm., and power densities above 50 Megawatts per square cm. Obviously, such high power density beams compound beam optics and tolerances problems, and the risk of rapid tube disintegration and burnout is greatly increased in proportion to the increased beam power densities.
It would accordingly be highly desireable to provide a permanent magnet focused tube, particularly one operable in the millimeter wave range at high power, whose beam diameter could be precisely adjusted during operation to optimize performance and avoid the above problems. However, although the above problems impeding progress have certainly been recognized, this has apparently not led to recognition of the desireability of an adjustable-feature tube, let alone an actual design for a tube with such characteristics and one which would alleviate the above problems. This is despite the fact that the art shows many examples of tubes having beam directional deviation or convergence compensation, as well as solenoid-focused linear beam tubes with an auxiliary coil in the gun section in series with the solenoid to help improve beam optics.
Turning to some specific prior art examples, in U.S. Pat. No. 2,867,746, a magnetic lens incorporating a coil is placed just downstream of the electron gun about the neck of a solenoid-focused klystron tube, in order to compensate beam misalignment and to reduce scalloping. The electron gun itself is magnetically shielded and not under the influence of a magnetic field. Thus, it is not a confined-flow-focused gun, and is unaffected by these adjustments, which have essentially no effect on beam diameter.
In U.S. Pat. No. 3,259,790, radially moveable polepiece extensions are equalized with a fixed internal auxiliary polepiece downstream of the cathode to correct axial misalignment of the beam, and to adjust beam convergence in a solenoid-focused tube. However, any attempt to control beam diameter by means of the adjustments of this arrangement introduces an unacceptably large degree of beam scalloping, certainly in millimeter wave applications. In a similar context, shim members also have been inserted in the gun region as another axial correction expedient for the beam, but without effect on beam size.
In U.S. Pat. No. 3,331,984, an iron cylinder member, actually a portion of the focus electrode structure, is affixed within the cathode, inside the vacuum envelope, and in electrical and thermal communication with the electron emitting element, in order to increase the magnetic area convergence of the beam. However, this beam convergence compensation arrangement is not at all adaptable to beam adjustments of any kind, since it must be affixed within the vacuum envelope and hence, after assembly of the tube, cannot be modified in any way to adjust for any beam imperfections, just as would be the case for any other electrode. The high cathode temperatures at which the member in question must operate can threaten its magnetic qualities, and even if it were possible somehow to access the member, its operation at or near cathode potential, and the proximity of other structures, including the anode, would preclude any change in position because of the risk of arcing and the tight physical confines of the cathode. A further reference with similar characteristics, but intended only for low convergence gun applications, is U.S. Pat. No. 3,522,469.